The decorative part, such as any of watch external parts including a watchband, a watchcase, a bezel, a caseback, an intermediate clasp, a buckle and a winding stem (winding crown), a piercing jewelry, an earring, a finger ring, a spectacle frame, a pendant, a brooch, a necklace or a bracelet, has a layer structure using various materials.
For example, a stainless material, Ti, a Ti alloy, brass and the like being soft materials whose machining into parts is easy are widely employed as a base material. However, it is pointed out that the decorative parts obtained by machining these soft base materials are likely to suffer degradation of the external appearance quality thereof due to marring during use, etc., posing a grave problem. The cause thereof is principally the lowness of the surface hardness of the soft base materials per se, for example, Hv=200 or so in terms of Vickers hardness. With a view toward solving this problem, various surface hardening treatments are being tested.
Further, for the above-mentioned decorative parts, such as any of watch external parts including a watchband, a watchcase, a bezel, a caseback, an intermediate clasp, a buckle and a winding stem (winding crown), a piercing jewelry, an earring, a finger ring, a spectacle frame, a pendant, a brooch, a necklace and a bracelet, high embellishing performance and external appearance quality are demanded. Accordingly, especially various surface hardening treatment technologies ensuring a luxurious golden color tone are being tested.
The methods for hardening a soft base material include the method of forming a coating on the surface of the soft base material and the method of hardening the soft base material per se.
As the method of forming a golden coating on the surface of the soft base material, there can be mentioned, for example, wet plating and ion plating methods. Although especially, Au plating, Au—Ni plating, Au—Pd plating and the like are widely carried out for decorative parts, all the plated coatings are soft and do not succeed in resolving the marring during use. Further, with respect to ion plating, although there can be mentioned techniques of forming a titanium nitride coating or the like, the titanium nitride coating has the drawback of, due to its high internal stress, having poor adhesion and tending to suffer peeling, and the techniques without exception do not succeed in completely overcoming the coating peeling problem. Moreover, in the event of peeling, there would occur exposure of the soft material per se where corrosion would occur to thereby disenable use as the part.
Although ion injection, ion nitriding, gas nitriding and the like are known as providing the method of hardening a decorative part consisting of a material, such as a stainless material, Ti, a Ti alloy or brass, all such means would need not only prolonged hardening treatment time leading to poor productivity but also high treatment temperature so that crystal grains of the decorative part would coarsen resulting in surface roughening to thereby markedly degrade the external appearance quality thereof. Especially, the problem of surface roughening is serious. When the decorative part having undergone mirror surface polishing treatment is subjected to gas nitriding or carburizing treatment, the crystal grains of the decorative part surface after the treatment would be coarse so as to induce a surface roughening of 200 to 300 μm. The mirror surface would disappear, and even when post-polishing is performed, the mirror surface condition before the carburizing treatment could no longer be recovered to thereby cause marked drop or deterioration of the external appearance quality thereof.
Furthermore, for providing a golden decorative part, it is proposed to superimpose an Au alloy top layer (finish layer) consisting of three elements, particularly 94.0 to 98.5 wt. % Au, 1.0 to 3.0 wt. % Fe and 0.5 to 3.0 wt. % Pd, on a nitride of at least one element selected from among Ti, Zr, Hf and Ta (see, for example, the following patent literature (1) or patent literature (2)). (The above is the background art of the first invention.)
As aforementioned, decorative parts, such as a watchband and other watch parts, a finger ring, a necklace and an earring, are provided at the outermost layer thereof with a gold coating by a wet or dry plating method to thereby impart premium accents or excellent corrosion resistance thereto.
As a decorative part exhibiting a golden color tone, there is disclosed a golden decorative part comprising a base material, a Ti coating containing unavoidable components (nitrogen, oxygen and carbon) formed on a surface of the base material in an inert gas other than nitrogen and an outermost layer coating formed on the Ti coating by a dry plating method wherein the outermost layer coating contains 60 to 99 at % gold, 0.5 to 20 at % iron and 0.5 to 20 at % unavoidable components (see patent literature (3)). In this prior art, the thickness of the outermost layer coating is generally in the range of 0.05 to 0.5 μm. It is also disclosed that it is preferred from the viewpoint of adhesion between the outermost layer coating and the base material that the thickness of the Ti coating generally fall within the range of 0.1 to 10 μm and that use be made of a TiN coating, HfN coating or the like. Further, it is disclosed that a Ti coating containing unavoidable components being present in a dry plating apparatus, formed in an inert gas other than nitrogen, such as argon, helium or neon, may be present as an undercoat layer (primer layer) between the base material and the TiN coating. (The above is the background art of the second invention.)    Patent literature (1): Japanese Patent No. 3244952    Patent literature (2): JP 2001-294955 A    Patent literature (3): JP 07-11462 A
The technology described in the background of the first invention is directed to adding of Fe and Pd with a view toward obtaining a golden decorative part ensuring an Au color tone without inviting metal allergy and makes no mention of hardness and strength at all. The writers' follow-up test revealed that only a coating whose hardness is as low as Hv 250 or below can be obtained. Accordingly, the object of the first invention is to, with respect to a decorative part based on a soft base material, such as a stainless material, Ti, a Ti alloy or brass, provide a decorative part of golden color tone having a high hardness and high marring resistance capable of preventing marring during use and having a high corrosion resistance capable of preventing any corrosion.
In the technology described in the background of the second invention, a gold coating and a coating of gold alloy, such as gold-iron alloy, pose the problem of generally having a low hardness to thereby tend to suffer marring and spoil the beauty of decorative part. Further, when the thickness of gold alloy coating (outermost layer coating) is as large as 0.05 to 0.5 μm like that of the above-mentioned decorative part, any blemish generated in the coating would be deep so as to be easily visible to the naked eye, resulting in marked spoilage of the beauty of decorative part. Accordingly, the object of the second invention is to provide a golden decorative part that, even when a golden outermost layer coating is marred, would suppress the visibility of blemish to the naked eye, thereby ensuring retention of the golden beauty over a prolonged period of time, and to provide a process for producing the golden decorative part.